Search

Recent simulation studies have provided evidence that a pulsating aurora (PsA) associated with high-energy electron precipitation is having a clear local impact on ozone chemistry in the polar middle mesosphere. However, it is not clear if the PsA is frequent enough to cause longer-term effects of measurable magnitude. There is also an open question of the relative contribution of PsA-related energetic electron precipitation (PsA EEP) to the total atmospheric forcing by solar energetic particle precipitation (EPP). Here we investigate the PsA-EEP impact on stratospheric and mesospheric odd hydrogen, odd nitrogen, and ozone concentrations. We make use of the Whole Atmosphere Community Climate Model and recent understanding on PsA frequency, latitudinal and magnetic local time extent, and energy-flux spectra. Analysing an 18-month time period covering all seasons, we particularly look at PsA-EEP impacts at two polar observation stations located at opposite hemispheres: Tromsø in the Northern Hemisphere (NH) and Halley Research Station in the Southern Hemisphere (SH). We find that PsA EEP can have a measurable impact on ozone concentration above 30 km altitude, with ozone depletion by up to 8 % seen in winter periods due to PsA-EEP-driven NOx enhancement. We also find that direct mesospheric NOx production by high-energy electrons (E> 100 keV) accounts for about half of the PsA-EEP-driven upper stratospheric ozone depletion. A larger PsA-EEP impact is seen in the SH where the background dynamical variability is weaker than in the NH. Clearly indicated from our results, consideration of polar vortex dynamics is required to understand PsA-EEP impacts seen at ground observation stations, especially in the NH. We conclude that PsA-EEP has the potential to make an important contribution to the total EPP forcing; thus, it should be considered in atmospheric and climate simulations. [ABSTRACT FROM AUTHOR]

Understanding the atmospheric forcing from energetic particle precipitation (EPP) is important for climate simulations on decadal time scales. However, presently there are large uncertainties in energy flux measurements of electron precipitation. One approach to narrowing these uncertainties is by analyses of EPP direct atmospheric impacts and their relation to measured EPP fluxes. Here we use observations from the microwave limb sounder (MLS) and Whole Atmosphere Community Climate Model (WACCM) simulations, together with EPP fluxes from the Geostationary Operational Environmental Satellite (GOES) and Polar-orbiting Operational Environmental Satellite (POES) to determine the OH and HO2 response thresholds to solar proton events (SPEs) and radiation belt electron (RBE) precipitation. Because of their better signal-to-noise ratio and extended altitude range, we utilize MLS HO2 data from an improved offline processing instead of the standard operational product. We consider a range of altitudes in the middle atmosphere and all magnetic latitudes from pole to pole. We find that the nighttime flux limits for day-to-day EPP impact detection using OH and HO2 are 50–130 protonscm-2s-1sr-1 (E>10 MeV) and 1.0– 2.5×104 electronscm-2s-1sr-1 (E = 100–300 keV). Based on the WACCM simulations, nighttime OH and HO2 are good EPP indicators in the polar regions and provide best coverage in altitude and latitude. Due to larger background concentrations, daytime detection requires larger EPP fluxes and is possible in the mesosphere only. SPE detection is easier than RBE detection because a wider range of polar latitudes is affected, i.e., the SPE impact is rather uniform poleward of 60∘ , while the RBE impact is focused at 60∘. Altitude-wise, the SPE and RBE detection are possible at ≈ 35–80 and ≈ 65–75 km , respectively. We also find that the MLS OH observations indicate a clear nighttime response to SPE and RBE in the mesosphere, similar to the simulations. However, the MLS OH data are too noisy for response detection in the stratosphere below 50 km , and the HO2 measurements are overall too noisy for confident EPP detection on a day-to-day basis. [ABSTRACT FROM AUTHOR]

Recently, a model for medium‐energy (30–1000 keV) radiation belt‐driven electron precipitation (ApEEP) has been put forward for use in decadal to century‐long climate model runs as part of the Climate Modelling Intercomparison Project, phase 6 (CMIP6). The ApEEP model is based on directly observed precipitation data spanning 2002–2012 from the constellation of low‐Earth‐orbiting Polar Operational Environmental Satellites (POES). Here, we test the ApEEP model's ability using its magnetic local time variant, ApEEP_MLT, to accurately represent electron precipitation fluxes from the radiation belts during a large geomagnetic storm that occurred outside of the span of the development data set. In a study of narrowband subionospheric very low frequency (VLF) transmitter data collected during March 2015, continuous phase observations have been analyzed throughout the entire St. Patrick's Day geomagnetic storm period for the first time. Using phase data from the U.K. transmitter, call‐sign GVT (22.1 kHz), received in Reykjavik, Iceland, electron precipitation fluxes from L = 2.8 to 5.4 are calculated around magnetic local noon (12 MLT) and magnetic midnight (00 MLT). VLF‐inferred >30‐keV fluxes are similar to the equivalent directly observed POES fluxes. The ApEEP_MLT >30‐keV fluxes for L < 5.5 describe the overall St. Patrick's Day geomagnetic storm‐driven flux enhancement well, although they are a factor of 1.7 (1.3) lower than POES (VLF‐inferred) fluxes during the recovery phase. Such close agreement in >30‐keV flux levels during a large geomagnetic storm, using three different techniques, indicates this flux forcing are appropriate for decadal climate simulations for which the ApEEP model was created. Key Points: Remarkably reliable subionospheric VLF transmitter phase measurements provide >30‐keV electron precipitation fluxes for March 2015VLF‐inferred >30‐keV electron precipitation fluxes are similar to the equivalent POES >30‐keV loss‐cone fluxes in the same regionCMIP6 > 30‐keV electron precipitation fluxes are only 1.3 times lower than the VLF‐inferred fluxes during the 2015 St. Patrick's Day storm [ABSTRACT FROM AUTHOR]

This study presents a new ion-neutral chemical model coupled into the Whole Atmosphere Community Climate Model (WACCM). The ionospheric D-region (altitudes ~50-90 km) chemistry is based on the Sodankylä Ion Chemistry (SIC) model, a one-dimensional model containing 307 ion-neutral and ion recombination, 16 photodissociation and 7 photoionization reactions of neutral species, positive and negative ions, and electrons. The SIC mechanism was reduced using the simulation error minimization connectivity method (SEM-CM) to produce a reaction scheme of 181 ion-molecule reactions of 181 ion-molecule reactions of 27 positive and 18 negative ions. This scheme describes the concentration profiles at altitudes between 20 km and 120 km of a set of major neutral species (HNO3, O3, H2O2, NO, NO2, HO2, OH, N2O5/ and ions (O+2, O+4, NO+, NO+ (H2O), O+2 (H2O), H+(H2O), H+(H2O)2, H+(H2O)3, H+(H2O)4, O-3, NO-2, O-, O2, OH-, O-2 (H2O), O-2 (H2O)2, O-4, CO-3, CO-3 (H2O), CO-4, HCO-3, NO-2, NO-3, NO-3 (H2O), NO-3 (H2O)2, NO-3 (HNO3/, NO-3 (HNO3/2, Cl-, ClO-/, which agree with the full SIC mechanism within a 5% tolerance. Four 3-D model simulations were then performed, using the impact of the January 2005 solar proton event (SPE) on D-region HOx and NOx chemistry as a test case of four different model versions: the standard WACCM (no negative ions and a very limited set of positive ions); WACCM-SIC (standard WACCM with the full SIC chemistry of positive and negative ions); WACCM-D (standard WACCM with a heuristic reduction of the SIC chemistry, recently used to examine HNO3 formation following an SPE); and WACCMrSIC (standard WACCM with a reduction of SIC chemistry using the SEM-CM method). The standard WACCM misses the HNO3 enhancement during the SPE, while the full and reduced model versions predict significant NOx, HOx and HNO3 enhancements in the mesosphere during solar proton events. The SEM-CM reduction also identifies the important ion-molecule reactions that affect the partitioning of odd nitrogen (NOx/, odd hydrogen (HOx / and O3 in the stratosphere and mesosphere. [ABSTRACT FROM AUTHOR]